Army ant description

The army ant Eciton burchellii is best known for its spectacular ‘swarm raids’, in which several thousand ants swarm across the ground and occasionally up into the forest canopy in search of prey (3). This ant is considered a ‘keystone species’ in many Neotropical rainforests (4), meaning that it plays a critical role in the rainforest ecosystem. Over 300 recorded species, from birds to insects, are known to associate with Eciton burchellii and are thought to depend on the ant for their existence in some way (3). In South and Central America, for example, around 50 bird species depend on this army ant species to flush their insect prey out of the leaf litter during the ant’s daily raids (4).

Eciton burchellii forms enormous colonies, with some colonies being made up of over half a million individuals. Each colony consists of permanently wingless queens and four worker castes. The smallest two castes, known as ‘minims’ and ‘mediums’, form small teams to collect prey items, while the larger ‘submajor’ caste are a specialised ‘porter’ caste that use their long legs to straddle prey items beneath their bodies. The largest caste of workers, known as ‘majors’, have the role of defending the colony from attack using their vicious mandibles(3)(5).

Related species

Army ant biology

The swarm raids of Eciton burchellii are the largest of any Neotropical army ant, with over 200,000 individuals proceeding at a rate of up to 20 metres per hour (6). The foraging workers spread out in a fan-shaped swarm with a broad front (6) and the ants maximise their foraging efficiency by forming ‘traffic lanes’, destroying all arthropods that occur in their path. The lanes are separated by different pheromone trails, which are laid down by the outgoing and incoming ants (7).

Army ant workers form super-efficient teams to carry large prey items (8) and will even plug holes with their bodies to increase the flow of ants across rough terrain (9). Using such ingenious methods, the ants are able transport more than 3,000 prey items per hour, in raiding columns that can exceed 100 metres in length (7). In the course of a day’s raiding, a single Eciton burchellii colony is able to capture around 30,000 prey items (4).

Eciton burchellii has a fixed 35-day activity cycle of raiding and migration that is associated with the growth of new broods of workers in the colony (10). For 20 days, the colony remains in a fixed bivouac at a single nest site, and will only raid every few days. During this time, which is known as the ‘statary phase’, the queen will produce up to 100,000 eggs (4)(6).

The eggs of Eciton burchellii develop in synchrony and hatch into larvae at the end of the 20-day statary period. The colony then moves into a ‘nomadic phase’ which lasts for 15 days. During this nomadic phase, when the colony has larvae to feed, the ants perform mass swarm raids every day and migrate to a new nest site each night (6). At the end of the 15 days the larvaepupate, and the colony once again enters the statary phase (4).

Species with a similar range

Army ant habitat

Eciton burchellii creates temporary nests, known as ‘bivouacs’, which are usually sited in fairly exposed positions such as in brush piles, between the buttresses of trees, on tree trunks, beneath fallen logs, or inside logs and hollow trees (3)(6).

The bivouacs are made up of the bodies of the ants themselves, with the workers hooking the claws on their legs to one another to encompass the queen and her eggs in the centre of the mass (3)(6).

Army ant status

Army ant threats

Tropical rainforests are being destroyed at an increasing rate and habitat degradation is therefore the most significant threat to Eciton burchellii(9).

Some forms of sustainable management are thought to negatively affect this important army ant species, as the felling of long strips of forest can create clusters of habitat which may be too small or inaccessible to maintain a large colony. Such practices may result in the army ant becoming locally extinct in some parts of its range (4).

Similarly, studies on Eciton burchellii have suggested that habitat corridors (strips of land connecting fragmented reserves) may also be detrimental to this species’ populations, unless the corridors are large enough for the army ant colonies to find and navigate successfully. If the army ant’s remaining habitat becomes too small, or the corridors between habitats are not sufficiently large,colonies are highly likely to become trapped in a small pocket of rainforest and will rapidly deplete the local resources, leading to starvation (4).

Army ant conservation

There are currently no specific conservation measures in place for Eciton burchellii.

Research on Eciton burchellii has suggested that this species would benefit from a sustainable forest-harvesting strategy where reasonably large, square-like clumps of forest are removed, rather than long, thin strips (4).

Glossary

Arthropods

A major grouping of animals that includes crustaceans, insects and arachnids. All arthropods have paired jointed limbs and a hard external skeleton (exoskeleton).

Caste

In social insect colonies, a group of individuals that are structurally and/or behaviourally distinct, performing certain tasks. Examples are the soldier caste of termites and ants, and the workers of bees.

Larvae

Immature stage in an animal’s lifecycle, after it hatches from an egg and before it changes into the adult form. Larvae are typically very different in appearance to adults; they are able to feed and move around but are usually unable to reproduce.

Mandibles

The pair of mouthparts most commonly used for seizing and cutting food, common to the centipedes, millipedes and insects.

Pheromone

A chemical produced by an animal, which stimulates a behavioural or physiological response by another member of the same species.

Pupate

The process of becoming a pupa, the stage in the life cycle of some insects during which the larval form is reorganised into the adult form. The pupa is usually inactive, and may be encased in a chrysalis, cocoon or other protective coating.

References

Franks, N.R., Sendova-Franks, A.B. and Anderson, C. (2001) Division of labour within teams of New World and Old World army ants. Animal Behaviour, 62: 635-642.

Rettenmeyer, C.W., Rettenmeyer, M.E., Joseph, J. and Berghoff, S.M. (2011) The largest animal association on one species: the army ant Eciton burchellii and its more than 300 associates. Insectes Sociaux, 58: 281-292.

Franks, N.R., Sendova-Franks, A.B., Simmons, J. and Mogie, M. (1999) Convergent evolution, superefficient teams and tempo in Old and New World army ants. Proceedings of the Royal Society B – Biological Sciences, 266: 1697-1701.

Partridge, L.W., Britton, N.F. and Franks, N.R. (1996) Army ant population dynamics: the effects of habitat quality and reserve size on population size and time to extinction. Proceedings of the Royal Society B – Biological Sciences, 263: 735-741.

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